A Leap Day

Today is February 29th, that odd date that only occurs every four years.

The reason for a leap day inserted into the calendar, the existence of February 29th, is ultimately astronomical. Perhaps a little explanation is in order…

We originally defined days as the time it takes the Earth to rotate. While we define years as the time it takes the Earth to orbit once around the Sun. The problem is that these values do not divide evenly into one another.

Mauna Kea Sunrise
Sunrise seen from the summit of Mauna Kea
The Earth takes about 365.24219 days to obit the Sun, when measured by the Sun’s position in the sky, what is called a tropical year. There are different ways to measure a year, but if one is concerned with keeping the seasons in sync with your calendar, then you are interested in tropical years.

It is that bunch of decimals, the 0.24219 etc., that is the problem, every four years the count drifts out of sync by roughly one day. The insertion of an extra day every four years helps bring the calendar back into synchronization with the orbit of the Earth and with the seasons.

Even leap years do not quite fix the problem as 0.24219 is close, but not quite 0.25 or one quarter of a day. Thus additional corrections are needed… Enter leap centuries.

Our current calendar was instituted by Pope Gregory XIII in 1582, setting up a standard set of corrections for the fractional difference between the length of a year and the length of a day. Scholars knew that errors had been accumulating in the calendar for centuries, resulting in a drift of several days. Religious authorities were concerned that this drift had displaced important celebration in the church calendar, in particular the celebration of Easter. After much argument it was decided to reform the calendar. The current solution was devised by a number of astronomers, including Aloysius Lilius, the primary author of the new system.

The Gregorian Calendar uses an extra day in February every four years, unless the year is divisible by 100, then there is no leap leap day that year. However, if the year is divisible by 400, then it is a leap year. While this may sound odd, it does create a correction much closer to the ideal value of 365.24219 days per year.

Even this is not perfectly precise. The correction is close but will drift given enough time. The length of a tropical year also changes slowly over time. We will eventually have to add another correction to keep the calendar and the seasons in sync. But not for a few millennia, good enough, for now.

As 2012 is divisible by four, there will be a leap day added to the end of this February… Today.

MOSFIRE Arrives at Keck

W. M. Keck Observatory press release…

A 10,000-pound package was delivered on Feb. 16 to the W. M. Keck Observatory near the summit of Mauna Kea. Inside is a powerful new scientific instrument that will dramatically increase the cosmic data gathering power of what is already the world’s most productive ground-based observatory.

The new instrument is called MOSFIRE (Multi-Object Spectrometer For Infra-Red Exploration). It is the newest tool to survey the cosmos and help astronomers learn more about star formation, galaxy formation and the early universe. The spectrometer was made possible through funding provided by the National Science Foundation and a generous donation from astronomy benefactors Gordon and Betty Moore.

“This is a crucial and important step,” said MOSFIRE co-principal investigator Ian McLean of U.C. Los Angeles, who has been involved in the building of four instruments for the Keck telescopes. “Just shipping it to Hawaii is the first step.” A long series of installation steps are already underway that will lead up to MOSFIRE’s “first light” on the sky and handover to the Keck community in August.

Hauling MOSFIREThe truck carrying MOSFIRE was escorted by police, Mauna Kea rangers and Keck Observatory personnel as it climbed the last few thousand feet to the summit. Photo by Larry O’Hanlon

MOSFIRE will gather spectra—chemical signatures in the rainbows of light from everything from stars to galaxies—at near-infrared wavelengths (0.97-2.45 microns, or millionths of a meter). That’s light which is beyond the red end of a rainbow—just a bit longer wavelength than human eyes can see. Observing in the infrared allows researchers to penetrate clouds of dust to see objects that are otherwise obscured. It also allows for the study of the most distant objects, the spectra of which have been stretched beyond optical wavelengths by the expansion of the universe.

What sets MOSFIRE apart from other instruments is its vastly more light-sensitive camera and its ability to survey up to 46 objects at once then switch targets in just minutes – an operation that takes comparable infrared instruments one to two days to complete.

“I reckon that MOSFIRE will observe very faint targets more than a hundred times faster than has ever been possible,” says Caltech astronomer Chuck Steidel, MOSFIRE’s co-principal investigator. “All the observations that my group and I have done in near-infrared spectroscopy with Keck over the last ten years could be done in just one night with MOSFIRE.”

Steidel anticipates that MOSFIRE will be one of the Keck’s workhorse instruments, used for about half of all telescope time on the Keck I Telescope. “It’s opening up a whole new area of study.”

Another big asset of MOSFIRE is that it can scan the sky with a 6.1 arc minute field of view, which is about 20 percent of a full moon and nearly 100 times bigger than the Keck’s current near-infrared camera. To take spectra of multiple objects, the state-of-the-art spectrometer consists of 46 pairs of sliding bars that open and close like curtains. Aligned in rows, each pair of bars blocks most of the sky, leaving a small slit between the bars which allow a sliver of light from the targeted object to leak through. Light from each slit then enters the spectrometer, which breaks down the object’s light into its spectrum of wavelengths.

MOSFIREMark Kassis stands beside the MOSFIRE spectrograph

Because everything that’s even somewhat warm radiates in the infrared, all infrared instruments must be kept cold to prevent any trace of heat from the ground, the telescope, or the instrument itself from messing up the signal from space, MOSFIRE is kept at a cool 120 Kelvins (about -243 degrees Fahrenheit or -153 degrees Celsius). This makes MOSFIRE the largest cryogenic instrument on the Keck telescopes.

Astronomers will use MOSFIRE to study the epoch of galaxy formation, as well as the so-called period of re-ionization, when the universe was just a half-billion to a billion years old. The instrument will also be used to investigate nearby stars, young stars, how stars formed, and even brown dwarfs, which are stars not quite massive enough for nuclear fusion to ignite in their cores.

MOSFIRE will also allow astronomers to do riskier—but more scientifically rewarding—research, Steidel says. Taking the spectrum of a single star or galaxy involves precious telescope time and resources. But because MOSFIRE can observe many objects at once, astronomers can afford to take extremely long exposures. Otherwise, such long exposures of single targets would be difficult to justify with limited telescope time and other observing targets waiting in line.

Caltech’s Keith Matthews, who has built two previous Keck instruments, plays a leading role as chief instrument scientist. The team includes the engineering and technical staff of W. M. Keck Observatory, the technical staff of the UCLA Infrared Lab, optical designer Harland Epps of UC Santa Cruz and the staff of Caltech Optical Observatories.

The Moon and Pleiades

Tonight a bright half Moon will be just under 4° from the Pleiades star cluster. The Moon will be 43% illuminated, bright, but the cluster is bright enough to be seen even against a bright Moon. As the Pleiades move to the west over coming months there will be a few more lunar conjunctions, with increasingly smaller crescents.

Deep Violet, an 18″ f/4.5 Dobsonian

In April 2001 I realized a dream that had been many years in the dreaming and a year in the making, a large aperture dobsonian.

Deep Violet
Deep Violet set up near the Dragoon Mountains in Southern Arizona
The decision process that eventually settled on the 18″ f/4.5 design was a long one. As a very active amateur I had had many opportunities to examine other scopes. To see where they excelled or where they fell short. With this experience I eventually decided on a list of requirements.

  • The scope had to have sufficient aperture to take advantage of the dark skies available near Tucson. I wanted to see spiral arms in galaxies.
  • The design was to be visual only. No drives, but provisions for an equatorial platform at a later date.
  • The mount would be a no compromise rigid structure, capable of allowing good optics to perform at their best.
  • The scope had to fit through a standard doorway.
  • The scope had to fit in the cargo compartment of a Ford Explorer Sport without dropping the seat for safety during transport.
  • The eyepiece must not be an excessive distance above the ground, allowing use while standing on the ground much of the time. (But then, I’m 6’2″ tall)

Over a decade of engineering experience has taught me that a well defined set of specifications can make all the difference at the end of a project. With these design goals in mind the plan then progressed rapidly.

Continue reading “Deep Violet, an 18″ f/4.5 Dobsonian”

Finding Venus in the Daytime

It is possible to see planets in the daytime. Both Jupiter and Venus are bright enough to see fairly easily in full daylight if conditions are right. You need to know where and how to look, but once glimpsed they are fairly easily seen. It is the knowing how that makes it possible. Try these simple hints…

  • Try when the planets are far from the glare of the Sun, in the first hours after dawn or last hours of the day are best.
  • Clean air is necessary. If the air is hazy, dusty or smoggy it will hide the planets from view, particularly when near the Sun. There will just be too much solar glare to pick out the planet. For the same reason try when the planet is high in the sky and you are looking through much less air.
  • The human eye will relax and defocus if there is nothing to focus on. This happens when looking at a plain expanse of blue sky. You could be looking right at the planet and not see it. A few puffy clouds around, or better yet, the Moon, will give the eye something to focus on, allowing the planet to be easily seen.
  • Put the Sun out of sight to reduce glare. Simply position yourself in the shadow of a tree or building to get a better view.
  • Pick a day when the Moon is near the object you are looking for, it will provide a simple signpost to the correct location.

It is this last hint that can be particularly useful today. Venus is about 4° degrees south of the Moon this afternoon. If you can find the Moon in the late afternoon check just below it and to the left for Venus. The planet will be about a eight lunar diameters away from the Moon, seen as a bright star-like object. Tomorrow it will be Jupiter’s turn, and you can try again to see a planet in the daytime.

Have a try.

Mars Opposition Reminder

One week from today, on March 3rd, the planet Mars will pass through opposition.

Mars orbits the Sun every 1.88 years, with Earth only taking one year for each orbit. Like two runners on a track the two planets race each other around the Sun. But we have the inside lane, lapping the red planet every two years. These events are called opposition, when Mars is closest to us and best positioned for viewing by earthbound telescopes.

Closest approach of the two planets is not necessarily on the same day as opposition, but can vary up to two weeks. This year closest approach will occur March 5th with the two planets approaching to 99,331,411 km (61,721,554 miles) at 07:01HST.1 At this distance the red planet will show a disk 13.89″ arc-seconds across in the eyepiece.2

Mars during the 2005 oppositionMars during the 2005 opposition

Do not worry about viewing on the 3rd or 5th, any time in the month leading up to and after opposition the viewing will be very good. With even a modest telescope it should be possible to see the bright polar caps and light and dark markings on the planet.

All month Mars will be visible throughout the night, high in the sky at midnight. This is the time to enjoy observing our closest neighboring planet while it is nearby and high in the night sky.

1) 2012 Mars Opposition at SEDS
2) The 2012 Aphelic Apparition of Mars

A Red LED Desk Lamp

Need a red lamp to provide soft red illumination around the telescope? Described here is an easy way to do this. Take a commonly available desklamp and convert it to LED’s. This is a single evening project, taking just a few hours to accomplish once the parts have been obtained.

Observing Table
A red LED desk lamp in use on the observing table

I have found the resulting light extremely useful when observing. No more holding the flashlight in your mouth when reading charts or making observing notes.

Because the light can light up a whole area when turned fully on it can be used as a worklight when breaking down gear and packing it into the vehicle without bothering other observers at a dark observing site. Or you might build the 120V version for a personal observatory.

I have also found that these lights also make great gifts for fellow astronomers.

To accomplish this we will disassemble the lamp, remove the 12V bulb and replace it with an array of LED’s. To control the brightness we will install a variable regulator to allow dimming of the lamp from completely off to very bright. An option is to install another connector to allow DC power to be supplied from a battery for field use.

Before embarking on this project be sure to read this entire article. Having the entire task in mind will help you plan ahead. It is important to use safe electronic assembly techniques to avoid any risk of damage or injury when making the modifications or when using the lamp. We will make no modifications to the high voltage side of the transformer to insure the lamp remains safe to use.

One secret to the lamp is the LED’s. These wonderful devices convert a small trickle of electricity into a very bright red light. Be sure to get ‘Ultrabright’ or ‘Superbright’ types that put out at least 3000mCd (millicandela). I found some that put out 6000mCd that I use in my lights at All Electronics. Perfboard, regulators and many of the other parts can also be purchased there.

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The Moon and Venus

Yesterday saw the Moon near Venus, tonight will see the pair even closer. It will be hard to miss the bright pair only 5° apart. Venus will be shining brilliantly at about -4.2 magnitude next to a 17% illuminated Moon. Jupiter can be seen 16° above the pair. Mercury is visible just above the sunset.